1. Field of the Invention
The present invention relates to an exposure technique used for fabricating semiconductor integrated circuits (ICs) and for fabricating masks for pattern transfer of the ICs and more particularly, to a charged-beam cell projection exposure system and a charged-beam cell projection exposure method using a beam of charged particles such as electrons or ions and an aperture mask with a plurality of different, shaped apertures for shaping the beam.
2. Description of the Prior Art
In recent years, to provide higher densities and higher operation speeds for semiconductor ICs, the dimensions of the devices and/or components in the semiconductor ICs have been continuously miniaturized.
To realize the miniaturization of the device/component dimensions, with the conventional optical exposure systems utilizing Ultra Violet (UV) light, such improvements as the use of light having a shorter wavelength, the increase in Numerical Aperture (NA), the modified illumination, and others have been made. At the same time, new types of the optical exposure method using the phase-shifting mask and others have been developed.
Further, the development of new types of the exposure other than the optical exposure, such as the an electron-beam or X-ray exposure, has been progressed.
For the semiconductor ICs having fine patterns, such as 256-Mbit Dynamic Random Access Memories (DRAMs), a variety of exposure techniques using an electron beam have been developed.
These exposure systems using the electron beam can be classified into two basic types: point beam and variable-shaped beam ones.
With the electron-beam exposure system of the point beam type, a pattern to be transferred on a resist-coated semiconductor wafer is divided into small unit regions, and the point beam of electrons is deflected for scanning, thereby writing the pattern on the resist-coated wafer in serial fashion.
On the other hand, with the electron-beam exposure system of the variable-shaped beam type, a pattern to be transferred is divided into unit rectangular regions. The electron beam giving a rectangular-shaped beam spot with a size and a shape corresponding to those of the unit regions is prepared and deflected for scanning, thereby writing the pattern on the resist-coated wafer in serial fashion.
Thus, these conventional electron-beam exposure systems require a long period of time for exposure. For example, the required exposure time for the above-mentioned 256-Mbit DRAMs is approximately as long as 10 minutes per chip. This is approximately 100 times the exposure time needed with the conventional optical exposure system using UV light.
Additionally, the conventional electron-beam exposure systems of these two types have a drawback that they are expensive compared with the conventional optical exposure systems.
To shorten the necessary exposure time, an improved electron-beam exposure method was developed, which was termed the "electron-beam cell projection exposure method".
This improved exposure method was disclosed in (a) the Japanese Non-Examined Patent Publication No. 52-119185 published in 1977, (b) the Extended Abstracts of the 50 th Autumn Meeting of the Japan Society of Applied Physics, 27a-K-6, pp. 452, 1989, entitled "Study of EB Cell Projection Lithography (I): Electron Optics", written by T. Matsuzaka et al., and (c) the Extended Abstracts of the 50 th Autumn Meeting of the Japan Society of Applied Physics, 27a-K-7, pp. 452, 1989, entitled "Study of EB Cell Projection Lithography (II): Fabrication of Si Aperture", written by Y. Nakayama et al..
In the improved exposure method termed the "electron-beam cell projection" exposure method, the pattern to be transferred is divided into repetitive unit or cell regions, and an aperture with a shape and a size corresponding to those of the cell regions is formed on an aperture mask. The electron-beam is shaped by passing through the aperture according to the unit regions, and is irradiated onto the resist-coated semiconductor wafer.
Specifically, an electron beam is irradiated to a rectangular first aperture formed on a first aperture mask, thereby shaping the beam into a rectangle. On the other hand, the pattern of a semiconductor chip to be transferred is divided into various repetitive cell regions, and various second apertures with sizes and shapes corresponding to those of the cell regions are formed on a second aperture mask. Any one of the second apertures is selectively used.
The rectangular-shaped electron beam by passing through the first aperture on the first aperture mask is irradiated to a selected one of the second apertures on the second mask, thereby forming a composite, shaped beam. The composite, shaped beam is then irradiated onto the resist-coated wafer, thereby making a spot with the composite shape on the wafer. Thus, the irradiated area of the wafer is exposed to the electron beam at a time.
The sizes of the second apertures are determined in such a way that the shaped spot formed on the wafer has a substantially uniform current density within the whole spot.
With the conventional electron-beam cell projection exposure method, the composite-shaped spot of the electron beam (i.e., the composite image of the first aperture and the selected one of the second apertures) is made on the resist-coated wafer at a single shot and therefore, the necessary exposure time for the chip pattern can be shortened.
An exposure system for performing the conventional "electron-beam cell projection" exposure method, which is capable of mass production, has been being developed, because the necessary electron optical system and the necessary aperture-mask configuration are able to be realized. An example of this system is shown in FIG. 1.
In FIG. 1, an electron beam 40, which has been emitted from an electron gun 41 as an electron source, is shaped into a rectangle by irradiating the beam 40 to a first aperture 43a on a first aperture mask 43. The rectangular-shaped beam 40 is then transmitted through a first shaping lens 44a, a deflector 45, and a second shaping lens 44b to be irradiated toward a second aperture mask 46 having shaped second apertures 46a.
The rectangular-shaped beam 40 is selectively irradiated to a selected one of the shaped second apertures 46a on the second aperture mask 46, thereby shaping the beam 40 into the shape of the selected second aperture 46a. Thus, the beam 40 is shaped into a composite shape of the first aperture 43a and the selected second aperture 46a.
The electron beam 40 with the composite shape is then reduction-projected onto a resist-coated semiconductor wafer 50 by means of a reduction lens 49a and a projection lens 49b. Thus, an image of the composite shape of the first and second apertures 43a and 46a is made on the electron resist film (not shown) on the wafer 50.
With the conventional electron-beam cell projection exposure system of FIG. 1, however, there is a problem that no fine pattern may be able to be formed on the resist film on the wafer 50. This is caused by the fact that the opening-area ratio of the selected one of the second apertures 46a maybe excessively large depending upon the type of the pattern to be transferred and as a result, the blur of the electron beam 40 with the composite shape will increase up to a level where the effects of the blur cannot be ignored due to the Coulomb interaction effect.
It is known that the strength of the Coulomb interaction effect increases with the increasing magnitude of the current density of the transmitted electron beam 40. Therefore, the above problem relating to the fine patterns can be solved if the current density of the beam 40 is designed to be adjusted in such a way that the beam 40 passing through a particular one of the second apertures 46a with the maximum opening-area ratio has a sufficiently low blur.
However, when exposing a cell region of the pattern to be transferred with a sufficiently-low opening-area ratio, it is preferred that the current density is as large as possible, because the necessary exposure time can be shortened. The shortened exposure time improves the total processing capacity of the exposure system of FIG. 1.
Therefore, to improve the total processing capacity of the conventional system of FIG. 1, it is necessary to adjust or change the current density of the electron beam 40 according to the opening-area ratio for the cell regions to be transferred.
To realize the current-density adjustment of the electron beam 40 according to the opening-area ratio for the cell regions to be transferred, an exposure system disclosed in the Japanese Non-Examined Patent Publication No. 4-137520 published in 1992 is available.
FIG. 2 shows the conventional exposure system disclosed in the Japanese Non-Examined Patent Publication No. 4-137520.
In the exposure system of FIG. 2, an electron beam 50, which has been emitted from an electron gun 51, is shaped into a rectangle by irradiating the beam 50 to a first rectangular aperture on a first aperture mask 53. The rectangular-shaped beam 50 is then transmitted through a first shaping lens 54a, a deflector 55, and a second shaping lens 54b, and is irradiated toward a second aperture mask 56 having shaped second apertures. The above configuration is the same as that of the conventional exposure system of FIG. 1.
However, a third aperture mask 66 and driving motors 67 are additionally provided in the system of FIG. 2 between the second shaping lens 54b and the second aperture mask 56.
The third aperture mask 66 has a plurality of meshed third apertures 66a, and serves to adjust the current density of the electron beam 50. The third apertures 66a of the third aperture mask 66 have different-sized meshes for the current densities from 100% to 50% in decrements of 10%. The motors 67 serve to translate the mask 66 in a plane perpendicular to the electron beam 50 in order to select one of the meshed apertures 66a.
The rectangular-shaped beam 50 having passed through the selected one of the third apertures 66a on the third aperture mask 66 is selectively irradiated to a selected one of the shaped second apertures on the second aperture mask 56, thereby shaping the beam 50 into a composite shape of the first aperture and the selected second aperture.
The electron beam 50 with the composite shape and the adjusted current density is then reduction-projected onto a resist-coated semiconductor wafer 60 by means of a reduction lens 59a and a projection lens 59b. Thus, an image of the composite shape of the first and second apertures is made on the resist (not shown) on the wafer 60.
In the Japanese Non-examined Patent Publication No. 4-137520, it is not explicitly stated that at which stage the selection of a particular one of the meshed third apertures 66a is determined. It is supposed that the determination of the apertures 66a is made at the stage of preparing the exposure data for the shaped apertures on the second aperture mask 56 according to their opening-area ratios.
Further, in the Japanese Non-examined Patent Publication No. 4-137520, it is disclosed that the meshed apertures 66a on the third aperture mask 66 may be directly formed on the first or second aperture masks 53 or 56. In this case, it is necessary to determine the proper mesh size and the layout of the apertures 66a prior to formation of the first or second aperture mask 53 or 56, respectively.
As described above, the conventional exposure system of FIG. 2, which was disclosed in the Japanese Non-Examined Patent Publication No. 4-137520, has the following problems:
First, various meshed apertures 66a need to be prepared on the corresponding aperture mask 66 according to the whole adjusting range of the current density.
Second, it is disclosed in the Japanese Non-examined Patent Publication No. 4-137520 that the meshes for adjusting the current density are designed to have a linewidth of 2.5 .mu.m or less. However, considering the further miniaturization of the patterns to be transferred in the future, the meshes need to have the linewidth of 1 or 0.5 .mu.m or less. Such extremely fine meshes are very difficult to be fabricated consistently, which substantially lowers the fabrication yield and greatly increases the fabrication cost.
Third, to automatically select a proper one of the current-density adjusting apertures 66a when the shaped apertures on the second aperture mask 56 are changed or replaced to be used from one to another, the determination or selection of the apertures 66a for adjusting the current density of the electron beam 50 needs to be made at the stage of preparing the exposure data for the shaped apertures on the second aperture mask 56 according to their opening-area ratios.